This application is a continuation-in-part of my U.S. patent application Ser. No. 06/595,756, filed on Apr. 2, 1984, and entitled "Transducer Coupling".
Circuits for deriving an automatic gain control voltage having an amplitude corresponding to the variations in the amplitude of the peaks of an alternating current wave generally utilize rectifying circuits having a diode connected between a source of the alternating current wave and a parallel combination of a capacitor and a resistor. When the amplitude of successive peaks of the wave is increasing, the capacitor is charged by current flowing through the diode so as to increase the voltage across the capacitor; and when the amplitude of successive peaks is decreasing, the capacitor is discharged by charge leaking through the resistor so as to decrease the voltage across the capacitor Whereas the time constant of the charging circuit can be made so short as to enable the voltage across the capacitor to follow rapid increases in the amplitude of the peaks, it is necessary for the time constant of the discharging circuit, which is the product of the resistance of the resistor and the capacitance of the capacitor, to be a compromise between the value required to permit the voltage across the capacitor to follow a sharp decrease in the amplitude between successive peaks and the value required to prevent a significant drop in voltage across the capacitor between peaks in a situation where the amplitude of the peaks remains constant. Thus, the automatic gain control voltage across the capacitor can follow sudden increases in ampliutde of the wave but does not follow sudden decreases in its amplitude as well.